The present invention relates to testing of optical components in particular for communication systems.
Measurement of polarization dependent parameters like polarization dependent loss (PDL) and polarization dependent group delay PDGD (covering Differential Group Delay DGD and Polarization Mode Dispersion PMD) is of increased importance for advanced communication systems and generally described in xe2x80x98Fiber Optic Test and Measurementxe2x80x99 by Dennis Derickson, ISBN 0-13-534330-5, 1998, pages 354ff. Especially long-haul high-speed systems require that polarization properties of its components fulfill certain requirements. In general, component manufacturers address this by 100% testing of components for critical parameters. PDL nowadays in many cases is already measured 100%, PDGD may also develop to be a 100% test in manufacturing.
Today""s solutions for measuring polarization dependent loss parameters are the scrambling method (applying a random variation of polarization states and comparing maximum with minimum determined loss) or the Mueller method, whereby 4 defined polarization states are measured for each wavelength point and analyzed together. The latter requires multiple measurement sweeps at predefined polarization states. These methods are either slow, if testing at multiple wavelengths is required (PDL measurement using the scrambling method), or require multiple measurement sweeps at predefined polarization states (Mueller method). Multiple sweeps are disadvantageous because measurement time is increased and require very high stability of the measurement setup because no change of polarization properties of the whole setup (between laser and DUT) is allowed between the sweeps.
It is an object of the invention to provide an improved measurement of polarization dependent parameters. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
For measuring polarization dependent parameters of an optical device under test (DUT), an optical source (preferably a tunable laser) provides an optical signal through an optical polarization translator to the DUT. The polarization translator translates the polarization of the optical signal from its input to its output in a deterministic way dependent on the wavelength of the optical signal.
The polarization translator provides the translation of the polarization dependent on the wavelength preferably using birefringent properties. Accordingly, the optical source may also provide a variation of the wavelength over the time, and polarization translator provides a variation of the polarization over the time, so that effectively the translator also provides a xe2x80x98translationxe2x80x99 of the polarization dependent on the wavelength. The parameters wavelength and frequency shall be regarded here as equivalents (related by the general equation f=c/xcex).
When varying the wavelength of the optical source, the polarization translator changes the polarization of the signal launched into the DUT. Tuning the wavelength of the optical source in a way that measurement points with different polarization states are covered thus allows determining polarization dependent parameters of the DUT in that particular wavelength range.
Typical polarization dependent parameters that can be analyzed by the invention are polarization dependent loss (PDL) or polarization dependent group delay PDGD (also referred to as Differential Group Delay (DGD) or Polarization Mode Dispersion (PMD)).
The uncertainty of the polarization state of the output signal may be reduced by tapping off some fraction of the signal in an appropriate way and analyzing its polarization state at each wavelength with a polarimeter or a reduced polarization analysis device like an Analyzer.
The polarization translator may be purely passive. The optical signal is preferably provided that it does not hit a Principle State of the Polarization (PSP) of the polarization translator, so that the output signal will follow a trajectory (e.g. a circle) on the Poincare Sphere in a deterministic way.
The same principle of scanning the polarization can be applied to various PMD measurement techniques: For example the Jones Matrix Eigenanalysis (JME) or a novel method which is outlined in the European Patent Application No. 125089.3 (EP 1113250). In case of PMD measurements in general only two polarization states are combined to a measurement value.
In case that several measurement points (defined by the wavelength and the polarization state of the optical signal applied to the DUT) are to be analyzed together for determining a value of a polarization dependent parameter, the wavelength range for those measurement points is preferably selected that a value of the polarization dependent parameter of the DUT can be considered as substantially constant in that wavelength range.
Preferred algorithms for analyzing together several such measurement points are interpolation of neighboring measurement points, combining 4 measurement points using the Mueller Matrix analysis, or combining 2 measurement points using e.g. the Jones Matrix analysis.
The invention has various advantages compared to today""s standard methods (polarization scrambling and Mueller Matrix analysis). The polarization transformation device may be purely passive, the number of measurement points can be chosen to be much smaller compared to the scrambling method, and, most important, the complete measurement can be performed within one sweep (instead of four for the Mueller Matrix Analysis). Thus, the invention allows fast measurements and is also less sensitive against e.g. environmental or mechanical disturbances.
The invention can be partly or entirely embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit.